Heart failure (HF), obesity, and diabetes mellitus (DM) often co-exist and are associated with the development and progression of cardiovascular disease, functional disability, increased symptoms, and diminished quality of life (QOL). Nutritional management of HF, obesity, and DM is poorly understood and little guidance is available regarding dietary approaches for these patients. The proposed study is designed to test the effects of a high protein (HP) diet vs. a standard protein (SP) diet on adiposity, cardiac structure, functional status, lipid profiles, glycemic control, insulin resistance, and QOL in 90 patients with HF, New York Heart Association class II and III, body mass index (BMI) 30 kg/m2, and DM. A prospective, two-group, randomized design will be used to test the study hypotheses. Participants will be randomized to either a HP or SP diet. The prescribed HP diet consists of 30% of energy from protein (~110 g/day), 40% of energy from carbohydrate (CHO; ~150 g/day) and 30% from fat (~50 g/day), while the SP diet consists of 15% of energy from protein (~55 g/day), 55% of energy from CHO (~200 g/day) and 30% from fat (~50 g/day). Patients in the 2 study arms will participate in an intensive 12-week supervised weight loss intervention during which they will receive personalized nutritional counseling and support at baseline and asked to return at 1, 2, 4, 8, and 12 weeks. Patients will be encouraged to engage in 20-30 minutes of physical activity, such as walking, three to five days per week and gradually increase to 30-60 minutes daily to promote energy deficit and facilitate weight loss. After the 3-month dietary intervention, participants will be instructed to continue the same dietary pattern followed during the previous 12 weeks of the study (maintenance). They will be seen every 3 months for one year (weeks 24, 36, 48, and 60) to monitor weight maintenance. Weight, BMI, waist circumference, dual-energy x-ray absorptiometry, 2D and Doppler echocardiography, cardiopulmonary exercise stress testing, six minute walk test, biochemical analyses (plasma lipid profiles, fasting blood sugar, and homeostasis model assessment), and self-reported measures of QOL will be completed at 3 measurement periods: baseline, immediately after the 3-month dietary intervention (12-weeks), and one year post intervention (60 weeks). The overall goal of the study is to test the hypothesis that a HP diet will result in significantly greater reductions in adiposity and greater improvements in cardiac structure, functional status, lipid profiles, glycemic control and insulin resistance, and QOL following the 3-month intervention and that these changes will be maintained over time. Mixed models will be used to analyze the data. Data from the proposed study will provide researchers and clinicians with a better understanding of the biobehavioral underpinnings associated with obesity, HP diets, and weight loss outcomes. This information will provide evidence for rational recommendations for weight loss modalities that can be integrated in HF management and treatment guidelines.